Sequential combustion gas turbines are known which include a compressor for compressing a main air flow. Such turbines can include a first burner for mixing a first fuel with the main air flow and generating a first mixture to be combusted, a high pressure turbine where the gases coming from the first burner are expanded, a second burner where a second fuel is injected in the already expanded gases to generate a second mixture to be combusted, and a low pressure turbine where also the gases coming from the second burner are expanded.
The second burner of the sequential combustion gas turbine can include a tubular body with a trapezoidal cross section.
The body can house, downstream of an inlet for the gas flow, four tetrahedral in shape vortex generators, arranged to generate four pairs of counter rotating vortices.
The vortex generators can be located at the upper, bottom and side walls of the body and, specifically, the upper and bottom vortex generators can be closer to the inlet of the body than the side vortex generators.
In addition, the upper and bottom vortex generators can have trailing edges which lay in a first plane perpendicular to the longitudinal axis of the burner, and the side vortex generators have trailing edges which lay in a second plane perpendicular to the longitudinal axis of the burner, the first plane being closer to the inlet than the second plane.
The burner can also include a lance to inject a fuel into the main compressed air flow, such that the fuel mixes with the compressed air and generates a mixture to be burnt.
The lance can be made of a number of coaxial tubular elements for injecting a liquid fuel, a gaseous fuel and air. Each of these tubular elements can be provided at the end of the lance with nozzles, which are coaxial with each other and define a plurality of nozzle groups for injecting fuel and air into the burner.
These nozzle groups can be all placed in a plane (the injection plane) and inject fuel along this injection plane.
The injection plane can be very far away from the second plane containing the trailing edges of the side vortex generators.
In addition, the nozzles groups can also be symmetrically placed both with respect to a transversal plane of the terminal portion of the lance and a longitudinal plane perpendicular to the transversal plane.
These features can allow an easy and inexpensive manufacturing of the burner and the lance, but can result in an incorrect mixing of the fuel with the hot gas flow coming from the high pressure turbine.
The quality of mixing can greatly influence the NOx emissions (according to an exponential correlation between NOx and unmixedness). It is therefore desirable to optimize the burner and, in particular, the lance which injects the fuel, in order to optimize mixing of the fuel with the main flow of compressed air and thus lower NOx emissions.